Why the size of atom does not decrease regularly in aperiod?

The size of atoms does not decrease regularly across a period due to the competing effects of increasing nuclear charge and electron shielding. As you move from left to right across a period, the number of protons increases, leading to a greater positive charge in the nucleus, which attracts electrons more strongly. However, the added electrons occupy the same principal energy level and do not significantly increase shielding, resulting in a more effective pull on the electrons and a decrease in atomic radius. This trend can be influenced by factors like electron-electron repulsion and the presence of d or f orbitals, leading to irregularities in atomic sizes.